Obama and Lynch will do what he can to cover for her, because the idea of Donald Trump (and more importantly the GOP) taking over the White House would likely result in the wiping out of a lot of Obama's legacy. But there is only so much cover Obama can give, before it backfires and threatens to take him down with her. If the Trump Sanders debate goes forward, it makes Hillary look weaker and vulnerable. If she looks like she is going to lose anyway, or would be a weaker candidate than Sanders, why would Democrats stick their necks out for her? They might as well take their chances with Bernie. And let's face it, Obama does not like the Clintons.